1. Field of Invention
The invention relates to a method and a device for regulating the metal ion concentration in an electrolyte fluid. The method and the device may particularly be used for regulating the copper ion concentration in a copper deposition solution that serves to electrolytically deposit copper and that additionally contains Fe(II) and Fe(III) compounds.
2. Brief Description of the Related Art
When the electroplating process is performed using insoluble anodes, it must be made certain that the concentration of the ions of the metal to be deposited is kept as constant as possible within the electrolyte fluid. This may be achieved by compensating for the loss of the metal ions in the electrolyte fluid, which is caused by the electrolytic deposition of metal, by adding the corresponding metal compounds for example. However, the supply and disposal costs for this method are very high. Another well-known method for complementing the metal ions in the electrolyte fluid consists in dissolving metal directly in the fluid with the help of an oxidizing agent such as oxygen for example. For copperplating, metallic copper can be dissolved in an electrolyte fluid that has been enriched with atmospheric oxygen. In this case, ballast salts, resulting among others from the complementation with metal salts, do not enrich in the electrolyte fluid. However, in the process of electroplating, oxygen is produced in both cases at the insoluble anodes of the electrolytic cell. This oxygen attacks the organic additives in the electrolyte fluid, these additives being usually added to the electrolyte fluid for controlling the physical properties of the deposited metal coating. Additionally, the oxygen also causes the anode material to be destroyed by corrosion.
In order to avoid the formation of noxious gases such as e.g., oxygen at the insoluble anodes and by using typical sulfuric acid copperplating baths that additionally contain chloride ions, as well as of chlorine, DD 215 589 B5 proposes a method for the electrolytic deposition of metal with insoluble anodes that consists in adding substances of an electrochemically reversible redox system as additives to the electrolyte fluid, Fe(NH4)2(SO4)2 for example, these substances being brought, by means of an intensive forced convection with the electrolyte fluid, to the anodes, where they are electrochemically converted by the electrolytic current, upon which conversion they are led, by means of intensive forced convection, away from the anodes into a metal ion generator in which they are electrochemically converted back to their original state on regeneration metal contained in said generator while, concurrently, the regeneration metal dissolves without the help of external current and, in their original state, they are returned to the deposition tank by means of intensive forced convection. The metal ions resulting from the dissolution of metal pieces in the metal ion generator are conveyed to the electroplating plant together with the electrolyte fluid.
In this process, noxious by-products are prevented from forming at the insoluble anodes. Additionally, the metal ions that have been used up in the electrolytic deposition of metal are subsequently produced by the reaction of the appropriate metal pieces with the substance of the electrochemically reversible redox system by causing the metal pieces to oxidize with the oxidized substances and the metal ions to form.
DD 261 613 A1 describes a method that uses, for the electrolytic copper deposition, substances of an electrochemically reversible redox system such as Fe(NH4)2(SO4)2 wherein it indicates that organic additives which are customarily utilized in the deposition fluid for the deposition of smooth and high-gloss copper coatings are not oxidized at the insoluble anodes while conducting the method.
DE 43 44 387 A1 also describes a method for the electrolytic deposition of copper with predetermined physical properties using insoluble anodes and a copper ion generator arranged outside the electroplating cell as well as substances of an electrochemically reversible redox system in the deposition fluid, the copper ion generator serving as a regeneration space for the metal ions and containing pieces of copper. It indicates that the organic additives contained in the deposition fluid have been observed to decompose while conducting the processes described in DD 215 589 B5 and DD 261 613 A1 so that, as a result thereof, in a deposition bath being in use for a longer period of time, decomposition products of these additives would enrich in said bath. To overcome this problem it suggests to use the substances of the electrochemically reversible redox system in a concentration that precisely leads to maintaining the total content of copper required for electroplating in the electroplating plant and to conduct the electrolyte fluid inside and outside the electrolytic cell in such a manner that the life of the ions of the reversible convertible substance that have been formed by oxidation at the anodes of the electrolytic cell is so limited in time in the overall electroplating plant that these ions are prevented or at least drastically hindered from destroying the additives.
The problem with the methods and devices mentioned is that the metal content in the electrolyte fluid cannot be kept constant easily. As a result thereof, the conditions for deposition vary, thus rendering it impossible to achieve reproducible conditions for the electrolytic deposition. One of the causes for the modification of the metal content in the electrolyte fluid is that the metal pieces in the metal ion generator are not only formed under the influence of the substances of the electrochemically reversible redox system, but also, in the case of a copper deposition bath using Fe(II)/Fe(III) compounds as substances of the electrochemically reversible redox system, by the oxygen from the air contained in the electrolyte fluid.
Moreover, it has also been found out that the oxidized substances of the electrochemically reversible redox system are not only reduced in the metal ion generator but also at the cathode in the precipitation tank, so that the cathodic current efficiency merely amounts to approximately 90%.
On account of the reasons mentioned above, a stationary condition between the formation of metal ions in the metal ion generator and the consumption of the metal ions by way of electrolytic metal deposition does not arise. This effect is still reinforced, specifically when using a higher temperature. Therefore, the content of the metal ions to be deposited in the electrolyte fluid increases continuously. However, the content of the metal ions has to be kept within narrow limits in order to keep up enough good physical properties of the deposited coatings of metal.
Among other indications, WO 9910564 A2 asserts in this connection that it is not possible to lower the metal ion concentration in the electrolyte fluid in an additional electrolytic secondary cell utilizing an insoluble anode in a manner which is well-known in conventional electroplating plants utilizing soluble anodes instead of the insoluble anodes employed here. The problem herewith, according to said document, is that the substances of the electrochemically reversible redox system are oxidized at the anode of the secondary cell so that the content of the oxidized species of these substances rises in the fluid. It maintains that, as a result thereof, the metal ion content in the electrolyte fluid continues to rise so that the actual goal aiming at lowering the metal ion concentration is missed.
The document mentioned additionally indicates another approach in overcoming the problem that involves diluting permanently the electrolyte fluid. But since this would entail that large quantities of the fluid would continuously have to be discarded and disposed of, this procedure, which is also known under the name of, feed and bleed method, is said to be unsatisfactory.
According to this document, the solution of the problem consists in suggesting a method and a device for regulating the metal ion concentration. According to this solution, at least one portion of the electrolyte fluid contained in the electroplating plant is guided through one or several electrolytic auxiliary cells provided with at least one insoluble anode and at least one cathode and a flow of current is set between the anodes and the cathodes of the auxiliary cells, said flow of current being so high that the current density at the surface of the anode amounts to at least 6A/dm′ and the current density at the surface of the cathode to no more than 3 A/dm? The ratio of the surface of the anodes to the surface of the cathodes is set to at least 1:4.
By means of this arrangement the metal ion content in the electrolyte fluid can be kept constant over a longer period of time by allowing part of the oxidized species of the electrochemically reversible redox system contained in the electrolyte fluid to be reduced at the cathode of the auxiliary cell. In adjusting the ratio of the current densities at the anode and at the cathode in the auxiliary cell by selecting for example the suitable relationship between the surfaces of the anode and of the cathode, the reduced species of the electrochemically reversible redox system at the anode of the auxiliary cell are oxidized merely to a minor extent or not at all so that the concentration of the oxidized species of the electrochemically reversible redox system can be regulated, which permits to directly influence the rate of formation of the metal ions.
The device described in WO 9910564 A2 proved however to be quite complicated since the precipitation tank has to be provided with several secondary cells. It is question of the auxiliary cell mentioned and of the metal ion generator. In production plants, it may be necessary to provide for a plurality of auxiliary cells and metal ion generators. Moreover, metal continuously deposits onto the cathode in the auxiliary cell so that the efficiency of the reduction of the oxidized species of the electrochemically reversible redox system continuously decreases at the cathode, thus requiring an increased electrical power. The rectifiers used for the purpose of supplying the auxiliary cell with current have to be provided with an increased rated capacity, which adds to the prime costs. Moreover, the duration of life of this device is limited on account of corrosive attack of the anode material.
Furthermore, the copper deposited on the cathode of the auxiliary cell has to be electrochemically removed from time to time which implies additional consumption of energy and non availability of the device for this period of time. Accordingly, several such auxiliary cells have to be provided to ensure continuous production, some of these cells being utilized for regulating the metal ion concentration while in other parallelled auxiliary cells the copper is being removed from the cathode. The particular disadvantage thereof is that the cathode material that is customarily employed is damaged in the stripping procedure. As a result thereof, the efficiency of reduction is reduced on one hand. On the other, the cathode has to be replaced by a new one after some stripping procedures.
Accordingly, the basic problem the present invention is dealing with is to overcome the drawbacks of the known methods and devices and to more specifically discover a device and a method that permit an economical way of operation of the procedure of electrolytic deposition. More specifically, the process of electrolytic deposition is intended to use insoluble anodes and substances of an electrochemically reversible redox system in the electrolyte fluid. The method is intended to be capable of being performed under constant conditions over a very long period of time. The metal ion concentration in the electrolyte fluid in particular has to be kept constant within narrow limits over said period of time. The invention is above all directed to permit to keep the metal ion concentration constant with simple means merely requiring low consumption of energy and low prime costs.